Field of the Invention
The field of the present invention is that of turbine engines and more particularly that of devices for reducing the noise produced by these turbine engines.
Description of the Related Art
Commercial aircraft are generally provided with turbofans, which are made up of a gas turbine driving a ducted fan, this being generally placed upstream of the engine. The air mass sucked in by the engine is divided into a primary stream, which circulates in the gas turbine or primary body, and a secondary stream, which comes from the fan, the two streams being concentric. The primary stream exits the fan in order to move into the primary body where it is compressed again, heated in a combustion chamber, guided to successive stages of turbines, then ejected in a primary gas stream. The secondary stream is compressed by the ducted fan stage, then ejected directly without having been heated. The two streams can be ejected separately in two concentric streams or mixed in a same channel prior to ejection.
The turbofan is conventionally housed in a nacelle which is shaped such as to make the aerodynamic drag as weak as possible; the nacelle comprises a first part, upstream, which surrounds the fan and a second part, downstream, which forms a fairing for guiding the secondary stream. The primary stream is, in the downstream part thereof, guided between a case of the engine, called a primary cowl, and a conical case which encloses the engine at the rear and which is generally called a tail cone. The fairing of the nacelle forms, with the primary cowl, an ejection nozzle for the secondary stream, whereas the first cowl forms, with the tail cone, an ejection nozzle for the primary stream.
The reduction of the noise caused by the jet of turbofans is a constant concern for aircraft manufacturers and engine builders and various solutions have been proposed therefor.
One of the current means used is chevrons which are installed on the primary nozzle of the engine. This technology is currently employed mainly on engines having separate streams. However, if it is quite effective from a sound point of view, it nevertheless has a negative effect on cruise performance.
Another solution envisaged by manufacturers consists in using micro-jets on the cowls surrounding the primary stream and/or the secondary stream. These micro-jets are circularly distributed in azimuth on the cowls and inject air into the corresponding jet, according to various angles of incidence and sideslip. However, studies carried out on various devices have shown that the sound gains which can be obtained by the control systems of this type remain limited, and this can be attributed to the lack of capacity for action of this method of control by external jets in the sound production areas, in this case downstream of the ejection nozzles, and on the dynamics of the flow coherent structures. Indeed, with devices located on the outside of the jet, in particular at the lip of the nozzle, the disturbance introduced is quickly assimilated by the turbulence of the layer where the two streams are mixed. The impact of this disturbance on the development of the turbulence depends then more on modifying the initial conditions of the mixing layer than on a direct action on the flow downstream areas, which is where the main sources of noise are located.